Epoxy resins are widely used, because of their nature, in printed wiring boards. It has been studied to design epoxy resins that are able to cope with the tendency of increase of mounting density of printed wiring boards which accompanies the recent achievement of reduction in size and weight.
Epoxy resins that have hitherto been proposed for this application include a cured product formed between 2,2-bis(34,-epoxycyclohexyl)propane and an acid anhydride (see patent document 1) and an epoxy resin compounded with a polyamide having a phenolic hydroxyl group in its carboxylic acid component (see patent documents 2 to 4).
These epoxy resins, especially those used in prepregs, are used as mixed with a large quantity of an inorganic filler of various kinds so as to reduce linear expansion thereby to control thermal deformation. However, incorporating a large amount of an inorganic filler can result in reduction in tensile strength or elongation, or the filler particles may agglomerate to cause a short-circuit, which will impair the circuit reliability.    Patent document 1: German Patent 1099733    Patent document 2: JP 2001-31784 (Claims and Examples from p. 6, left col., 1. 40 to p. 8, last line)    Patent document 3: JP 2001-49082 (Claims)    Patent document 4: JP 2005-29720 (Claims)